Dead forests release less carbon into atmosphere than expected

March 25, 2013
by Daniel Stolte

Vast swaths of forest are succumbing to pine beetle outbreaks. Credit: David Moore/UA School of Natural Resources and the Environment

(Phys.org) —Billions of trees killed in the wake of mountain pine beetle infestations, ranging from Mexico to Alaska, have not resulted in a large spike in carbon dioxide released into the atmosphere, contrary to predictions, a UA-led study has found.

Massive tree die-offs release less carbon into the atmosphere than previously thought, new research led by the University of Arizona suggests.

Across the world, trees are dying in increasing numbers, most likely in the wake of a climate changing toward drier and warmer conditions, scientists suspect. In western North America, outbreaks of mountain pine beetles (Dendroctonus ponderosae) have killed billions of trees from Mexico to Alaska over the last decade.

Given that large forested areas play crucial roles in taking carbon dioxide out of the atmosphere through photosynthesis and turning it into biomass, an important question is what happens to that stored carbon when large numbers of trees die.

Warmer wintertime temperatures are thought to boost pine beetle populations, while extended dry periods make trees more susceptible to the insects. Credit: David Moore/UA School of Natural Resources and the Environment

"The general expectation we had was that when trees die on a large scale, it would lead to a big pulse of carbon into the atmosphere through microorganisms metabolizing all that dead wood," said David Moore, an assistant professor in the School of Natural Resources and the Environment in the UA College of Agriculture and Life Sciences and one of the lead authors of the study, which is published online in the journal Ecology Letters.

"A question we are looking to answer is, 'How does the carbon dioxide released from the forest into the atmosphere change as you have large scale tree mortality over time?''' said second lead author Nicole Trahan, a postdoctoral researcher at the University of Colorado, Boulder.

According to co-author Russell Monson, who is the Louise Foucar Marshall Professor in the UA School of Natural Resources and the Environment, forests affect the carbon budget of the atmosphere through two dominant processes: photosynthesis, by which plants take carbon dioxide out of the atmosphere and lock it up in organic compounds, and respiration, by which plants and soil microbes release carbon dioxide back into the atmosphere. The balance of these processes determines whether a particular forest is a carbon source or a carbon sink.

After a massive tree die-off, conventional wisdom has it that a forest would go from carbon sink to carbon source: Since the soil microbes are still around, they are expected to release large amounts of the greenhouse gas carbon dioxide into the atmosphere, where it is thought to accelerate climate change.

"Surprisingly, we couldn't find a big pulse," said Moore, who is also a member of the UA Institute of the Environment.

Trahan added: "In the first few years after beetles have come in and killed trees, the carbon release from the surrounding soil actually goes down."

Large amounts of dead trees, it turns out, hold on to their carbon for a long time and prevent it from quickly being released into the soil or the atmosphere. According to Moore, this might be due to several reasons: First, while trees take up carbon dioxide during the day during photosynthesis, they release some of it at night when they switch to respiration.

"Once the trees are dead, respiration by the trees goes away," Moore said. "In addition, if you cut off the carbon that a tree put into the soil while it was alive, you reduce the ability of the soil microbes around the roots to respire."

"After five or six years, there is a buildup of some dead plant material, leaf litter and so on, and that seems to drive the rate of respiration up again. But it never recovers to the point it was before the beetles killed the trees, at least over the span of a decade," Moore said.

Finally, the trees studied in this project grow at higher elevations, where cooler temperatures slow the decomposition process and thereby carbon-releasing respiration.

Looking up into pine trees infested with pine beetles. Credit: David Moore/UA School of Natural Resources and the Environment

"Overall, we discovered that after a tree die-off, the loss of carbon in the soil results less from increased respiration by microbes but more from the fact that trees are no longer sequestering photosynthesized carbon into the soil," Moore said. "There seems to be a dampening of the carbon cycle rather than a big pulse of carbon release. So even if the forest now goes from a sink to a source of carbon dioxide, it's not as dramatic of an effect as we thought it would be."

The large areas of high-elevation forests are some of the most important carbon sinks for Western North America, Moore said. Worldwide, plants take about 120 billion tons of carbon out of the atmosphere each year. About half of that is released again through plant respiration, while the other half is released through respiration by animals, microbes and other organisms.

According to data published by the Intergovernmental Panel of Climate Change, or IPCC, burning of fossil fuels and land use change result in an annual net addition of about 8 billion tons of carbon dioxide to the atmosphere.

"The reason we study the natural carbon cycle is because any change, even a small one, could have a large effect on our climate," Moore said.

Outbreaks of mountain pine beetles are nothing new, Moore said.

"The beetles are a natural phenomenon, they have been there for very long time. Their populations go through periods of boom and bust. But extended drought hinders the trees ability to withstand the attacks."

In the past, cold temperatures used to kill beetles off in the wintertime, but with temperatures on the rise, this happens less and less. Add to that a climatic trend to more and longer droughts, and you have a double whammy, Moore explained.

"Over the past two decades, it has not been very cold in the wintertime and we have experienced a succession of very dry years across the Southwest, rendering the trees more susceptible to beetles."

For the study, the team set up experiments and collected data in two areas: Fraser Experimental Forest in Colorado's Arapaho National Forest in Colorado, and Niwot Ridge, a study site near Nederland, Colo.

"We measure the carbon dioxide coming out of the soil," Moore explained. "At the Fraser site, we used a method to measure the CO2 as it increased during the day and decreased overnight at the bottom of the valley. We used that as a measure of how much carbon dioxide was cycled across the entire valley."

Using a flux tower at Niwot Ridge, the team was able to measure the net uptake of carbon in the area. A flux tower monitors eddies, or pockets of air, with instruments that measure the levels of carbon dioxide and water, allowing researchers to estimate how much carbon the ecosystem is pulling in from the atmosphere versus how much water and carbon it is losing.

To put the data into a broader context, Moore and his colleagues compared the fluxes measured at Niwot Ridge to measurements obtained by NASA's Moderate Resolution Imaging Spectroradiometer aboard the Terra and Aqua Satellites. NASA provides a global dataset by measuring the amount of green vegetation and the temperatures on the Earth's surface.

"Those datasets are very good on a global level but less so for a given local area," Moore said. "Therefore, we used flux tower data to validate the NASA product for our region."

At Fraser Experimental Forest and the Niwot Ridge sites, the team took advantage of a sequence of trees in which time of death had been documented.

"This gives us a timeline of almost a decade where we know what happened to those trees," Trahan said. "We know when they died, and we can follow what happens to the carbon in the system."

"As long as a tree is alive, it puts much of the carbon it fixes from the atmosphere underground to support its roots and associated microorganisms," Trahan explained. "When the tree dies, that carbon flow is shut off, and the release of carbon into the soil and the atmosphere goes down, leading to the observed dampening effect on the carbon cycle: As trees die, less carbon is taken up from the atmosphere, but less is released from the soil as well."

The data obtained in studies like this one can be used to inform models that help scientists predict how ecosystems will respond to these disturbances in the future.

Related Stories

One helpful action anyone can take in response to global warming is to plant trees and preserve forests. Trees and plants capture carbon dioxide during photosynthesis, thereby removing the most abundant greenhouse gas from ...

Vast stores of carbon in U.S. forest soils could be released by rising global temperatures, according to a study by UC Irvine and other researchers in today's online Proceedings of the National Academy of Sciences in Washington, ...

(Phys.org)—Using dogwood trees, scientists are gaining a better understanding of the role photosynthesis and respiration play in the atmospheric carbon dioxide cycle. Their findings will aid computer modelers in improving ...

Recommended for you

Five million years ago, the Colorado River met the Gulf of California near the present-day desert town of Blythe, California. The evidence, say University of Oregon geologists, is in the sedimentary rocks exposed at the edges ...

Pressure, temperature and fluid composition play an important role in the amount of metals and other chemicals found in wastewaters from hydraulically fractured gas reservoirs, according to Penn State researchers.

Pioneering work being carried out in a cave in New Mexico by researchers at McMaster University and The University of Akron, Ohio, is changing the understanding of how antibiotic resistance may have emerged and how doctors ...

(Phys.org)—A team of researchers with the European Commission's Joint Research Centre and Google Switzerland has combined historical data with modern mapping engines to produce high-resolution maps of the world's surface ...

The ice sheet covering Greenland is four times bigger than California—and holds enough water to raise global sea-level more than twenty feet if most of it were to melt. Today, sea levels are rising and the melting of Greenland ...

23 comments

Great study! I like this work. AND please let us note that the article does not consider the influx of CO2 into the atmosphere that will happen when these BILLIONS of trees catch fire, as they almost inevitably will.

"Billions of trees killed in the wake of mountain pine beetle infestations, ranging from Mexico to Alaska, have not resulted in a large spike in carbon dioxide released into the atmosphere, contrary to predictions, a UA-led study has found. "

If the models are so good, and the science is so settled, why are envirosciences still not predicting things 100% correctly every time. After all, the entire definition of "settled science" would be a 100% accuracy and predictive power.

If the models are so good, and the science is so settled, why are envirosciences still not predicting things 100% correctly every time. After all, the entire definition of "settled science" would be a 100% accuracy and predictive power.

I'm surprised this has to be explained to you again .Exactitude is an underpinning of engineering, not science. No one has ever said the models are perfect triple, and your lack of understanding of science becomes transparent when you continually insist that they should be.

Along with that, you are again confusing minutia with an over all trend. Which, again, only goes to point out your misunderstanding of the science.

@ ScooterG - an idiotic comment as usual! Regardless, you should try flying over the middle of a forest where all you can see for miles in every direction is dead trees. Or drive through them. Or camp in them. Your flippancy would dry up pretty fast.

Grrr I hate responding to your garbage ScooterG, but you make such assinine statements I can't help myself! I've been in a forest fire where there were uncountable dead trees left behind by a pine beetle infestation, and watched as friends and neighbors lost their homes. Bull flipping crap anyone has said these trees are less prone to burning, and your ignorance of a forest eco-system is appalling!

Seriously, you are too stupid to be allowed to reproduce and cause a further shallowing of the human gene pool!

Do you understand what a "crown fire" is? Can you take the time to read through what you post, and then apply your brain cell to trying to comprehend what is said?

Yes, emo, I do understand. Did you read the (cited) article???

From that article:

"Meanwhile, the best defense for homeowners living in wildfire country is to clear away combustibles near the home, Kohler said. "It's all about fuel, whether it's alive or dead, and if you reduce the amount of fuel, crown fires are less likely to happen."

"Homeowners also are encouraged to thin trees around their property to ward off beetles by reducing competition among the trees, Kohler said."

Gee whiz...If we can ward off beetles and reduce fire risk by thinning around homes, why can we not ward off beetles and reduce fire risk by thinning the rest of the forest? Answer: ignorant radical enviro-idiots.

Wow triple. Ok Ì will type really slowly here, so you can start to understand.

The science of whether or not the global climate is warming is settled. It is.

The science behind the cause of the global climate warming is settled. It is the result of the release of GIGA tonnes of CO2 into the atmosphere by humans.

The science of the results of the two settled ideas above is not settled, because modelling of a complex system like Earth`s weather is exceptionally difficult. Ergo, the MINUTIA of the fine details of the resulting effects cannot be precisely determined through modelling.

The science of whether or not the global climate is warming is settled. It is.

The science behind the cause of the global climate warming is settled. It is the result of the release of GIGA tonnes of CO2 into the atmosphere by humans.

-- Maggnus

The science of the results of the two settled ideas above is not settled, because modelling of a complex system like Earth`s weather is exceptionally difficult and so the models do not match reality. But, really, it is the 0.28% man-made greenhouse gases that's responsible. Ergo f%$# your ignorant self.

Aww look at againstseeing, so cute trying to be all clever. I'm so impressed, you even managed to string togeather three sentences, almost all of which made sense! Keep following my lead, in a few years (given your painfully slow cognitive skills), you'll be sounding like a person with intellegence.

Ya good point, when you have the mentalitity of a 5 year old and your striving to reach the mentality of a grade fiver, and you've been through grade 3 6 times and still fail, it does become hard to imagine you ever achieving a degree of intellegence that would be noticable to adults.

Ya good point, when you have the mentalitity of a 5 year old and your striving to reach the mentality of a grade fiver, and you've been through grade 3 6 times and still fail, it does become hard to imagine you ever achieving a degree of intellegence that would be noticable to adults.

You keep on striving though, your mother/cousin still loves you!

-- magganusTurdAt least learn to spell - Intelligence, if you are going to use it to try and insult others. What a ReTurd.

At least learn to spell - Intelligence, if you are going to use it to try and insult others. What a ReTurd.

YAWN!! Ya whatever, bored with you now.

@ScooterG - Within some very narrow parameters, I owe you an apology. Trees denuded of their needles are, indeed, less prone to sudden flare ups, and in fact areas where all of the trees are dead and have been for some time, they will, in fact, act as a fire break, depending on the amount of debris left as deadfall.

@ScooterG - Within some very narrow parameters, I owe you an apology. Trees denuded of their needles are, indeed, less prone to sudden flare ups, and in fact areas where all of the trees are dead and have been for some time, they will, in fact, act as a fire break, depending on the amount of debris left as deadfall.

So, my apologies, I was wrong about that part.

Within some very narrow parameters, I have gained some respect for you. Apology accepted.

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.